No single material is suitable for all medical tubing because the specific requirements of medical applications vary greatly. It is often necessary to determine the requirements for each type of medical application and then develop a suitable material with appropriate properties. The main requirements for applications involving medical tubing are that the tubing exhibits chemical stability while in contact with body tissues and fluids, provokes little or no reaction by the patients body, and retains a capacity to deliver fluid flow when folded into tight bends. Physical properties such as strength, flexibility, hardness, and elasticity determine whether tubing is capable of delivering flow in specific conditions.
Medical tubing is used for a variety of applications, including drains, catheters, cannula, shunts, and replacements for natural tubular organs. Some of these devices are intended to remain in contact with or within the body for long periods. Others are used only briefly. Materials frequently used for medical tubing are polyethylene, polypropylene, polytetrafluoroethylene, polyvinylchloride, polydimethylsiloxane, polyurethane, and natural rubber.
Thermoplastic polyurethane elastomers are a special class of polyurethanes that soften or melt at elevated temperatures. When cooled, they recover their former physical and chemical properties. Thermoplastic polyurethane elastomers can be produced by the reaction of 4,4'-diphenylmethane diisocyanate (commonly called MDI), polyester or polyether polyols, and glycol extenders. Desired properties, such as tensile strength, elongation, and hardness, are obtained by adjusting the nature and the amount of the components, the length of polyurethane chains present in the elastomer, and the degree of cross-linking between such chains.
Representative of the thermoplastic urethane class of elastomers are the Estane.TM. elastomers manufactured by the B. F. Goodrich hemical Company and the Pellethane.TM. elastomers manufactured by the Dow Chemical Corporation. They are prepared from di- or polyisocyanates, such as MDI; glycols, polyols or multifunctional carboxylic acids, such as adipic acid; and 1,4-butanediols. Thermoplastic-thermosetting urethane polymers, such as the Texin.TM. polymers manufactured by Mobay Chemical Company, which may be extruded and injection molded, are Considered thermoplastic urethane elastomers. They are prepared from hydroxyl terminated polyethers, polyesters, MDI, or similar diisocyanates. The Pelletnane.TM. elastomers manufactured by the Dow Chemical Corporation are also classified as thermoplastic polyurethane elastomers.
While polyurethane elastomers have been utilized to manufacture medical tubing, it appears that none of the previously known tubing compositions possessed all of the physical properties which physicians seek in medical tubing. A good tubing should possess a relatively high degree of hardness, as it must often be guided through small passages to reach a location in the body where it can be put to use. It should also have a high degree of tensile strength, as mechanical failure during withdrawal of the medical tubing might cause serious complications.
At the same time, the ideal medical tubing has a sufficient degree of flexibility and elasticity, so as to conform to a patient's movements and cause no injury to the patient. Because the patient's movements tend to constrict or fold the medical tubing, it is important that the tubing continue to function as a conduit and maintain flow even while subject to external mechanical forces. Finally, when mechanical force is removed the tubing should substantially recover its original shape.